Apicomplexan parasites are among the most widespread human and animal pathogens. This group includes the causative agent of malaria (Plasmodium sp.), the opportunistic pathogen Toxoplasma gondii, and numerous important veterinary pathogens. In recent years, a novel organelle has been discovered within these parasites: a plastid acquired by secondary endosymbiosis of an alga, and retention of the algal plastid. The apicomplexan plastid, or "apicoplast", has emerged as a target for several promising anti-parasitic drugs. However, the essential metabolic function(s) of this organelle remain unclear. In order to design novel chemotherapeutic agents that target the apicoplast, it is essential to understand its function and role in pathogenesis. Because the apicoplast genome encodes no metabolic enzymes, elucidation of its function will require identification of nuclear-encoded proteins that are imported into the organelle. The aim of this proposal is to dissect the targeting signals employed by nuclear-encoded apicoplast proteins for translocation across the apicoplast membranes, defusing a novel organellar-targeting pathway. This work is also expected to facilitate the development of parasiticidal agents and drug delivery mechanisms. Information obtained from these studies will be integrated with ongoing computational studies seeking to identify novel drug targets and define a complete metabolic picture of the apicoplast. Specific aims are as follows: 1. Dissection of the Ferredoxin.-NADP Reductase (FNR) transit peptide. 2. Determination of the transit peptide cleavage motif.